Semiconductor memory device

Abstract: A semiconductor memory device comprising a flip-flop circuit (11); a switch (12) provided between the flip-flop circuit and a pair of data lines and a write circuit (16) writing data into the flip-flop circuit via the switch. The device further comprises potential shift means (44) connected to the pair of data lines, for generating potentials derived from potentials of the pair of data lines. This allows a higher reading speed to be obtained.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to semiconductor memory devices, and more particularly to control performed at the time of writing data into and reading data from a semiconductor memory device such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory). 2. Description of the Prior Art Recently, a semiconductor memory device has employed a relatively large chip and the data lines for transferring data read from memory cells to output circuits have become longer. Further, in order to increase the access speed, a complex read operation has been employed. Examples of such a complex read operation are a page mode and static column mode in the DRAM. In such a complex read operation, a case will frequently occur in which second data is read via a pair of data lines in a state in which first data remains in the pair of data lines. If the first read data and the subsequent second read data have opposite polarities of the pair of data lines, it takes a long time to inverse the polarities of the pair of data lines due to a parasitic capacitance coupled to the long data lines. In order to avoid the above problem, conventionally, the pair of data lines is set to constant potentials at the time of reading data from a memory cell. The constant potentials limit the amplitude voltage between the pair of data lines and contribute to reduction the time it takes to inverse the polarities of the pair of data lines. Fig. 1 is a circuit diagram of an essential part of a conventional semiconductor memory device. Referring to Fig. 1, m (m is an integer equal to or larger than 1) bistable flip-flop circuits 1 are coupled to a pair 3 of data lines D and /D via m selection transistor circuits 2. It will be noted that "/D" denotes an inverted version of a signal "D" or a signal line carrying the inverted signal /D. The symbol "/" corresponds to "bar" shown in the drawings. The above holds true for the combination of other characters with the symbol "/". A load circuit 4, a read buffer circuit 5 and a write buffer circuit 6 are connected to the pair 3 of data lines of a complementary type. Each of the flip-flop circuits 1 includes two N-channel MOS transistors (FETs) Q1 and Q2 and two P-channel MOS transistors Q3 and Q4, and stores write data or read data with complementary signals. Power supply voltages VCC and VSS (VCC > VSS) are supplied to each of the flip-flop circuits 1. For example, the power supply voltages VCC and VSS are 5V and 0V, respectively. Each of the selection transistor circuits 2 includes two N-channel MOS transistors (transfer gates) Q5 and Q6. The load circuit 4 includes two P-channel MOS transistors Q7 and Q8. The write buffer circuit 6 includes two P-channel MOS transistors Q9 and Q11, and two N-channel MOS transistors Q10 and Q12. When the circuit shown in Fig. 1 is provided in the DRAM, memory cells (not shown) are connected to the flip-flop circuits 1. When the circuit shown in Fig. 1 is provided in the SRAM, the flip-flop circuits 1 function as memory cells. The flip-flop circuits 1 may be used as sense amplifiers provided for respective pair of bit lines. Fig. 2 is a waveform diagram of the read operation of the circuit shown in Fig. 1. At the commencement of the read operation, a control signal / ("/" denotes the same meaning as that of bar shown in Fig. 1) is set to a low level, and the data lines D and /D are set to a high level (approximately equal to VCC). In the above state, a select signal m-2, for example, is switched to the high level. It will now be assumed that nodes N1 and /N1 of the flip-flop circuit 1 associated with the select signal m-2 are at the high level (H) and the low level (L), respectively. When the select signal m-2 is switched to the high level, the transistor Q5 and Q6 are turned ON, and thus the nodes N1 and /N1 become connected to the data lines D and /D, respectively. The data line D is continuously at the voltage VCC because the node N1 is at the high level, while the potential of the data line D starts to decrease because the node /N1 is at the low level. Then, the potential of the data line /D is settled to a limited potential depending on the ratio of the ON resistances (internal resistances) of the transistor Q2 of the (m-2)th flip-flop circuit 1, the transistor Q6 and the transistor Q7 of the load circuit 4. It will now be assumed that the above limited potential is denoted as VCC - α. Thereafter, it will be assumed that a select signal m-1 is switched to the high level at time t = 0, and the (m-1)th flip-flop circuit 1 is selected. It will be assumed that nodes N2 and /N2 of the (m-1)th flip-flop circuit 1 are at the low level (L) and the high level (H), respectively. Since the select circuit 2 is made up of the N-channel MOS transistors Q5 and Q6, the source potentials thereof must be equal to or lower than VCC - Vth (Vth is the threshold voltage of the N-channel MOS transistors) when the gate potential is VCC. The N-channel MOS transistor Q5 connected to the node N2 is strongly turned ON because the source potential thereof is VSS. The N-channel MOS transistor Q6 connected to the node /N2 is little turned ON because the source potential thereof is VCC - α. Hence, the (m-1)th flip-flop circuit 1 is influenced from only the data line D via the transistor Q5 connected to the node N2. Hence, the potential of the data line D is decreased to VCC - α, and the potential of the data line /D is increased to VCC. As described above, in the read operation, the potential difference between the pair of data lines is limited to a constant voltage (α in the above example) in the high-potential side of the amplitude voltage of the flip-flop circuit 1, so that the potential inverting speed can be facilitated. For example, the above potential difference α is approximately equal to 0.5 V. A description will now be given, with reference to Fig. 3, of the write operation performed subsequent to the above-mentioned read operation. It will now be assumed that the nodes N1 and /N1 of the (m-2)th flip-flop circuit 1 are at the high level (H) and the low level (L), respectively, as shown in Fig. 1. As shown in Fig. 3, the select signal m-2 is switched to the high level, and thus data stored in the flip-flop circuit 1 is read. Further, the potentials of the data lines D and /D are respectively VCC and VCC - α. Fig. 3 shows the waveforms obtained when the load circuit 4 is operated and the data line pair is connected to the VCC line. When data different from data stored in the (m-2)th flip-flop circuit 1 is written therein in the above state, write data signals Dw and /Dw are set to the low and high levels, respectively (write buffer activating operation). At this time, the driving signal / applied to the load circuit 4 is set to the low level as in the case of the read operation. The data line D is connected to the VSS line via the transistor Q12, and the data line /D is connected to the VCC line via the transistor Q9. The potential of the data line /D is rapidly increased to the voltage VCC, and the potential of the node /N1 is rapidly increased. After the write buffer activating operation, the potential of the data line D starts to gradually decrease. When the source voltage of the transistor Q5 becomes lower than the gate voltage thereof by the threshold voltage Vth, the transistor Q5 is turned ON. Then, the potential of the node N1 starts to decrease. Thereafter, the state of the (m-2)th flip-flop circuit 1 is inverted, and the write operation is completed. The potential of the data line D is settled to a level Vcc - β dependent on the ratio of the ON resistances of the transistors Q8, Q12, Q5 and Q1. However, the above-mentioned conventional semiconductor memory device has the following disadvantages. Fig. 4 is a circuit diagram of the configuration of the read buffer circuit 5 shown in Fig. 1. More particularly, the circuit shown in Fig. 4 is a current-mirror circuit made up of MOS transistors Q13 - Q22. The data lines D and /D are connected to the gates of the N-channel MOS transistors Q16 and Q20. Read data Dr and /Dr are respectively output via a node at which the transistors Q18 and Q20 are connected together and a node at which the transistors Q14 and Q16 are connected together. It is known that the current-mirror type read circuit as shown in Fig. 4 has the best AC input/output characteristic (operates at the highest speed) when inputs D and /D are approximately equal to VCC/2. However, the aforementioned conventional circuit configuration is designed to stabilize the operation of the flip-flop circuits 1 by precharging the data lines D and /D to the power supply voltage VCC. Hence, the read buffer circuit 5 provided in the conventional circuit configuration cannot be operated in the best AC input/output characteristic. That is, the read buffer circuit 5 operates at a speed lower than the maximum speed. Further, in the write operation described with reference to Fig. 3, the source potential of the transistor Q5 of the (m-2)th select circuit 2 remains at the voltage VCC for a while, and starts to decrease. Hence, it takes a long time for the transistor Q5 to be turned OFF, and the write operation cannot be performed at high speed. With the above in mind, it may be considered that the driving signal / applied to the load circuit shown in Fig. 1 is switched to the high level when data is written whereby the load circuit 4 is disconnected from the data lines D and /D. Fig. 5 is a waveform diagram showing the operation observed under the above consideration. At the time of the write operation, the driving signal / is switched to the high level in order to rapidly turn ON the transistor Q5 of the (m-2)th select transistor 2. In this case, the amplitude limitation regarding the data lines D and /D activated in the write operation is removed, and the amplitude on the pair 3 of data varies within the full range-, as shown in Fig. 5, as indicated by τ shown therein. Hence, it takes a long time to reset the data lines D and /D (set the potentials of the data lines D and /D to VCC) after the write operation. In the case shown in Fig. 5, it takes a long time to precharge the data line D to VCC. Furthermore, it is necessary for large current to flow in the circuit in order to precharge the data line D, and a large amount of energy is consumed. JP 05 028761A discloses a semiconductor memory device according to the preamble of accompanying claim 1. In this device, separate pairs of read data lines and write data lines are provided. A flip-flop circuit is not electrically connected to the write data lines, but is connected to the gates of switching transistors which are electrically connected to the write data lines. SUMMARY OF THE INVENTION An embodiment of the present invention can provide a semiconductor memory device in which the above disadvantages are eliminated. According to the present invention, there is provided a semiconductor memory device comprising a flip-flop circuit; a switch provided between the flip-flop circuit and a pair of data lines; a write circuit writing data into the flip-flop circuit via the switch; and potential shift means connected to the pair of data lines, for converting potentials of the pair of data lines into particular potentials, the potential shift means comprising a first and a second transistor; the first transistor has a gate connected to a first data line, a source receiving a first power supply voltage, and a drain outputting one of said particular potentials obtained by converting a potential of a second data line; the second transistor has a gate connected to the second data line, a source receiving said first power supply voltage, and a drain outputting another one of said particular potentials obtained by converting a potential of the first data line, said first and second data lines corresponding to said pair of the data lines; characterized in that said potential shift means further comprises a first series circuit having a first end connected to the first transistor, and a second end receiving a second power supply voltage; and a second series circuit having a first end connected to the second transistor, and a second end receiving the second power supply voltage, the first series circuit having first and second resistance elements connected in series, the second series circuit having third and fourth resistance elements connected in series, a node at which the first and second resistance elements are connected in series being connected to said second data line, and a node at which the third and fourth resistance elements are connected in series being connected to said first data line. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: Fig. 1 is a circuit diagram of an essential part of a conventional semiconductor memory device; Fig. 2 is a waveform diagram of the read operation of the circuit shown in Fig. 1; Fig. 3 is a waveform diagram of the operation of the circuit shown in Fig. 1 in a case where a power supply voltage VCC is applied to a pair of data lines in the write operation; Fig. 4 is a circuit diagram of a read buffer circuit shown in Fig. 1; Fig. 5 is a waveform diagram of the operation of the circuit shown in Fig. 1 in a case where the pair of data lines is disconnected from a VCC power supply in the write operation; Fig. 6 is a circuit diagram of a semiconductor memory circuit useful for understanding the present invention; Fig. 7 is a waveform diagram of the operation of the circuit shown in Fig. 6; Fig. 8 is a block diagram of a DRAM device to which the Fig. 6 circuit is applied; Fig. 9 is a block diagram of the DRAM device shown in Fig. 8 in more detail; Fig. 10 is a diagram of a configuration of a read operation control circuit shown in Fig. 9; Fig. 11 is a circuit diagram of a configuration of a sense amplifier driving signal generating circuit shown in Fig. 9; Fig. 12 is a circuit diagram of another semiconductor memory circuit useful for understanding the present invention; Fig. 13 is an equivalent circuit diagram of the circuit shown in Fig. 12 in the read operation; Fig. 14 is a waveform diagram of the read operation of the circuit shown in Fig. 13; Fig. 15 is a circuit diagram of an internal voltage generator shown in Figs. 12 and 13; Fig. 16 is a circuit diagram of a decoder circuit shown in Figs. 12 and 13; Fig. 17 is a circuit diagram of a conventional potential shift circuit; Figs. 18A and 18B are equivalent circuit diagrams of the conventional potential shift circuit shown in Fig. 41; Fig. 19 is a circuit diagram of an embodiment of the present invention; Figs. 20A and 20B are equivalent circuit diagrams obtained when a driving signal rises in the potential shift circuit according to the embodiment of the present invention; Fig. 21 is a block diagram of a DRAM device to which the embodiment of the present invention is applied; Fig. 22 is a block diagram of an SRAM device to which the embodiment of the present invention is applied; Fig. 23 is a circuit diagram of an essential part of the circuit shown in Figs. 21 and 22; Fig. 24 is a waveform diagram of the operation of the DRAM device shown in Fig. 21; Fig. 25 is a circuit diagram of another configuration of the potential shift circuit according to the present invention; Figs. 26A and 26B are equivalent circuit diagrams of the configuration shown in Fig. 25 obtained when a driving signal falls; and Figs. 27A and 27B are circuit diagrams of memory cells used in a DRAM device and an SRAM device, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 6 is a circuit diagram useful for understanding the invention, but not in accordance with the same. In Fig. 6, parts that are the same as those shown in Fig. 1 are given the same reference numbers. The semiconductor memory circuit of Fig. 6 includes a plurality of (m) flip-flop circuits 1, a pair 3 of data lines D and /D, select transistor circuits 2 selectively connecting the flip-flop circuits 1 and the data line pair 3 in response to a select signal 1 - m based on an external input address signal, a read buffer circuit 5 for amplifying data read to the data line pair 3 from one of the flip-flop circuits 1 via the associated select transistor circuit 2, a write buffer circuit 6 for writing one of the flip-flop circuits via the data line pair 3 and the associated select transistor circuit 2, and a load circuit 4A connected to the data line pair 3. The load circuit 4A functions to limit the voltage amplitude between the data lines D and /D to a voltage amplitude of the flip-flop circuits 1 or less, preferably approximately half the voltage amplitude of the circuits 1, at least when the data write operation is performed. For this purpose, the load circuit 4A functions to generate a predetermined voltage (clamp voltage) equal to approximately half the power supply voltage VCC. It will now be assumed that the nodes N1 and /N1 of the (m-2)th flip-flop circuit 1 are at the high level (H) and the low level (L). When the select signal m-2 is switched to the high level, high-level data appears on the data line D, and low-level data appears on the data line /D. In the read operation, the voltage amplitude between the data lines D and /D is limited to the above-mentioned voltage α. This voltage α depends on the ratio of the ON resistances of the transistors Q7, Q6 and Q2 and the ON resistances of the transistors Q8, Q5 and Q3. When the data read operation is performed, the data line pair 3 has been precharged to VCC/2. That is, when the data read operation is performed, the driving signal / is set to the low level. In this case, as shown in Fig. 7, since the node N1 is at the high level, the potential of the data line D becomes equal to VCC/2 + α/2, and the potential of the data line /D becomes equal to Vcc/2 - α/2. When inverse data is written into the (m-2)th flip-flop circuit 1 in the above state, the data signals Dw and /Dw are set to the low level and the high level, respectively. In the write operation, the driving signal / is maintained at the low level. Hence, the voltage VCC/2 is applied to the data lines D and /D. The data write circuit 6 has an internal resistance which causes the voltage amplitude between the data lines D and /D appearing at the time of the write operation falls in a predetermined limited range δ (for example, δ = 2α). More specifically, the . dimensions of the transistors Q9 - Q12, the above limited range δ is selected. When the write data different from the read data is written into the (m-2)th flip-flop circuit 1, the potential of the data line /D rapidly increases to VCC/2 + α, and the potential of the data line D rapidly decreases to VCC/2 - α. The transistors Q5 and Q6 are strongly turned ON in which the node /N1 functions as the source of the transistor Q6 and the data line D functions as the source of the transistor Q5. Hence, data can be written into the (m-2)th flip-flop circuit 1 from both the data lines D and /D while there is no need to stop the operation of the load circuit 4A by controlling the control signal /, so that data is written at high speed. Further, in the data write operation, the voltage amplitude between the data lines D and /D is limited and has the center equal to VCC/2. Hence, it becomes possible to rapidly reset the data lines D and /D after the write operation is completed. Further, the reset operation precharges the data line set to the high level in the write operation to VCC/2 and precharges the data line set to the low level in the write operation to VCC/2. Hence, the reset operation does not need much current as in the conventional case. At the commencement of the read operation, the data lines D and /D have been precharged to VCC/2, and the voltage amplitude between the data lines D and /D is limited to α. Hence, when the read buffer circuit 5 has the current-mirror type circuit configuration as shown in Fig. 4, the amplifying operation can be performed with the best AC input/output characteristic of the current-mirror type circuit, and data can be read at high speed. The circuit of Fig. 6 will now be described in more detail. The circuit configuration shown in Fig. 6 differs from that shown in Fig. 1 in that the data lines D and /D in the configuration shown in Fig. 6 are precharged to VCC/2 before the read operation or the write operation is performed. As shown in Fig. 7, before the read operation is started, the data lines D and /D are precharged VCC/2. Assuming that the (m-2)th flip-flop circuit 1 holds data shown in Fig. 6, when the select signal m-2 rises, the potential difference between the data lines D and /D is increased to a around the voltage VCC/2. When the write data different from the data stored in the (m-2)th flip-flop circuit 1 is written therein, the data signals Dw and /Dw are set to the low level and the high level, respectively. Thereby, the transistor Q9 is turned ON, and the power supply voltage VCC is applied to the data line /D. Further, the transistor Q12 is turned ON, and the power supply voltage VSS is applied to the data line D. Then, the transistors Q5 and Q6 of the select transistor circuit 2 associated with the (m-2)th flip-flop circuit 1 are strongly turned ON in the state in which the node /N1 functions as the source of the transistor Q6 and the data line D functions as the source of the transistor Q5. Hence, the potential of the node N1 is decreased, and the potential of the node /N1 is increased. At a certain time, the state of the (m-2)th flip-flop circuit 1 is inverted. A current flows to the power supply VCC/2 from the power supply VCC via the transistors Q9 and Q7, and a current flows to the power supply VCC from the power supply VCC/2 via the transistors Q8 and Q12. In this case, it is possible to set the amplitude of the data line /D to VCC/2 + α and set the data line D to VCC/2 - α by controlling the ON resistances of the transistors Q9, Q7, Q8 and Q12 by designing the dimensions of these transistors. Similarly, the dimensions of the transistors Q10 and Q11 are designed. Hence, the potential of the data line D can be rapidly decreased to VCC/2 - α, and the potential of the data line /D can be rapidly increased to VCC/2 + α. As a result of the above operation, the transistors Q5 and Q6 can be rapidly turned ON immediately after the write operation is commenced. It is desirable that the potential difference δ between the data lines D and /D obtained in the write operation is greater than the potential difference α obtained in the read operation (for example, δ = 2α). When data is read from the (m-2)th flip-flop circuit 1 according to the above-mentioned read operation, the potential of the data line D becomes equal to VCC/2 + α/2 and the potential of the data line /D becomes equal to VCC/2 - α/2, as shown in Fig. 7. When α is equal to, for example, 0.5 V, the potential of the data line D becomes equal to VCC/2 + 0.25 V, and the potential of the data line /D becomes equal to VCC/2 - 2.25 V. It will now be assumed that, at the above time, due to a cause, the level of the select signal m-2 decreases and the (m-1)th flip-flop circuit 1 is selected by the select signal m-1 in which circuit 1 the node N2 is at the low level (L) and the node /N2 is at the high level (H). In this case, if the amplitude between the data lines D and /D obtained in the write operation is equal to 0.5 V (= α), the situation observed when the (m-1)th flip-flop circuit 1 is selected is the same as that observed in the write operation. In order to distinguish the situation observed in the read operation from that observed in the write operation, the amplitude δ between the data lines D and /D obtained in the write operation should be greater than the amplitude α therebetween obtained in the read operation. For example, when α = 0.5 V, the amplitude δ should be set to a voltage greater than 0.5 V. Fig. 8 is a diagram of the structure of a DRAM equipped with the circuit shown in Fig. 6. A DRAM chip 100 includes a plurality of memory cells arranged in rows and columns, sense amplifiers 11, and column selecting gates 12. The memory cells 10 and the sense amplifiers 11 are connected, via the column selecting gates 12, to a pair of data buses formed with the data lines D and /D. The sense amplifiers 11 and the column selecting gates 12 correspond to the flip-flop circuits 1 and the select transistor circuits 2 shown in Fig. 6, respectively. A sense buffer 15 and a write amplifier 16 are connected to the data bus pair 13. The sense buffer 15 and the write amplifier 16 correspond to the read circuit 5 and the write circuit 6 shown in Fig. 6, respectively. An input/output circuit connected to the sense buffer 15 and the write amplifier 16 outputs input data (write data ) Din to the write amplifier 16, and outputs output data (read data) Dout from the sense buffer 15 to an external device. A DC load control circuit 14 generates the driving signal / from a related timing signal among timing signals output by a control circuit 18. The driving signal / is switched to the low level in the write and read operations. The driving signal / is applied to the gates of the P-channel MOS transistors Q7 and Q8. A VCC/2 voltage generator 17 generates VCC/2 from the power supply voltage VCC supplied from the outside of the DRAM device, and applies the voltage VCC/2 to the data bus pair 13. The transistors Q7 and Q8, the DC load control circuit 14, and the VCC/2 voltage generator 17 correspond to the load circuit 4A shown in Fig. 6. The control circuit 18 receives, from the outside of the DRAM device, a row address strobe signal /RAS, a column address strobe signal /CAS, and a write enable signal /WE, and generates various timing signals (control signals) necessary for the operation of the DRAM. In a case where there are a plurality of selectable bits, an output enable signal /OE is applied to the control circuit 18. A sense amplifier driving signal generating circuit 19 receives a related control signal from the control circuit 18, and generates sense amplifier driving signals (VSS, VCC) for the low and high level sides. The sense amplifier driving signals are applied in common to the sense amplifiers 11. An address input circuit 20 receives an address signal from the outside of the DRAM device, and outputs a row address signal to a plurality of row decoders 21 (only one row decoder is shown in Fig. 8 for the sake of simplicity), and a column address signal to a plurality of column decoders 22. Each of the column decoders 22 decodes the received column address signal, and generates a select signal applied to the associated column selecting gate 12. The row decoders 21 shown in Fig. 8 decodes the row address signal from the address input circuit 20, and selects one of a plurality of word lines 25 (only one word line is shown in Fig. 8 for the sake of simplicity) connected to the memory cells 10. A plurality of pairs 24 of bit lines are connected to the memory cells 10, as shown in Fig. 8. Fig. 9 shows the structure shown in Fig. 8 in more detail. In Fig. 9, parts that are the same as those shown in Fig. 8 are given the same reference numbers. The address signal from the outside of the DRAM device is input to an address input circuit/predecoder 26. A predecoded row address signal is decoded by a row decoder 27, which selectively word liens 25 arranged in a memory cell array 28. A predecoded column address signal is decoded by a column decoder 26, which selectively drives pairs 24 of bit lines arranged in the memory cell array 28 via a column select gate/sense amplifier circuit 27. The write data Din is written into a selected memory cell in the memory cell array 28 via a write data input circuit 23a , the write amplifier 16, the data bus pair 13, and the column select gate/sense amplifier circuit 27/ Data read from a selected memory cell in the memory cell array 28 is output, as data Dout, to the outside of the DRAM device via the column select gate/sense amplifier circuit 27, the data bus pair 13, a DC load/sense buffer circuit 29, and a data output circuit 23b. The voltage VCC/2 generated by the VCC/2 voltage generator 17 is applied to a DC load/sense amplifier circuit 29. A read operation limit circuit 34 and a DC load/sense buffer circuit 29 correspond to the DC load control circuit 14, the sense buffer 15 and the transistors Q7 and Q8 shown in Fig. 8. A /RAS (Row Address Strobe) operation control circuit 30, a /CAS (Column Address Strobe) operation control circuit 31, a write operation control circuit 32, a data input/output control circuit 33 and a read operation control circuit 34 shown in Fig. 9 correspond to the control circuit 18 shown in Fig. 8. The /RAS operation control circuit 30 outputs a control signal responsive to the fall of the row address strobe signal /RAS in the write operation and the read operation to the address input circuit/predecoder circuit 26, the row decoder 27 and the sense amplifier driving signal generating circuit 19. The /CAS operation control circuit 31 outputs a control signal responsive to the fall of the column address strobe signal /CAS in the write operation and the read operation to the write operation control circuit 32, the data input/output operation control circuit 33, the read operation control circuit 34 and the address input circuit/predecoder circuit 26. The write operation control circuit 32 drives the write data input circuit 23a, the write amplifier 16 and the read operation control circuit 34 when the write enable signal /WE falls and then the column address strobe signal /CAS falls. As shown in Fig. 10, the read operation control circuit 34 includes an inverter 34a. The inverter 34a receives, from the /CAS operation control circuit 31, the control signal which is switched to the high level in the column selecting operation, and outputs, as the driving signal /, the inverted version of the above control signal to the gates of the P-channel MOS transistors Q7 and Q8 of the DC load/sense buffer circuit 29. That is, the driving signal / is switched to the low level in the write and read operations. In Fig. 10, dots "·" denote input terminals (signals), and dots "o" denote output terminals (signals). This holds true for the following description. Fig. 11 is a circuit diagram of the sense amplifier driving signal generating circuit 19, which is comprised of two inverters INV1 and INV2, two P-channel MOS transistors Q23 and Q24, and two N-channel MOS transistors Q25 and Q26. A control signal generated by the /RAS operation control circuit 30 and switched to the high level in response to the fall of the row address strobe signal /RAS is applied to the gate of the transistor Q24 and the inverter INV2. A control signal (latch enable signal) switched to the high level in response to the rise of the word line is applied to the gate of the transistor Q26 and the inverter INV1. The voltage VCC/2 generated by the VCC/2 voltage generator 14 is applied to the gates of the transistors Q24 and Q25. Before the read and write operations, the row address strobe signal /RAS is at the high level, and hence the low-level control signal is applied to the transistor Q24 and the inverter INV2. Hence, the voltage VCC/2 (functioning as a signal PSA) is applied to the P-channel MOS transistors Q3 and Q4 (Fig. 6) of the sense amplifier (flip-flop circuit) 1 via the transistor Q24. Further, the voltage VCC/2 (functioning as a signal NSA) is applied to the N-channel MOS transistors Q1 and Q2 of the sense amplifier 1 via the transistor Q25. It will be noted that Fig. 6 shows that the constant power supply voltages VCC and VSS are applied to the sense amplifiers 1 for the sake of simplicity. In the read or write operation, the transistors Q24 and Q25 are turned OFF, and the transistors Q23 and Q26 are turned ON. Hence, the power supply voltages VCC is applied to the transistors Q3 and Q4, and the power supply voltage VSS is applied to the transistors Q1 and Q2. In another semiconductor memory circuit shown in Fig. 12, useful for understanding the invention but not in accordance with the same, a voltage switch 37 and an internal voltage generator 38 are added to the circuit configuration shown in Fig. 6. The internal voltage generator 38 generates an internal voltage VL (VSS < VL < VCC) from the power supply voltages VCC and VSS. The voltage switch 37 outputs either the power supply voltage VCC or the internal voltage VL in accordance with the level of the write enable signal /WE to a decoder circuit 39 (which corresponds to the column decoder in the DRAM device or a row decoder in a SRAM device). More particularly, the voltage switch 37 includes a P-channel MOS transistor Q51 and an N-channel MOS transistor Q51 and an N-channel MOS transistor Q52. The write enable signal /WE is applied in common to the gates of the transistors Q51 and Q52. In the write operation, that is, when the write enable signal is at the low level, the power supply voltage VCC is applied to the decoder circuit 39. In the read operation, that is, when the write enable signal /WE is at the high level, the internal voltage VL is applied to the decoder 39. Fig. 13 is an equivalent circuit of the circuit shown in Fig. 12 in the read operation. It will now be assumed that data is read from the (m-2)th flip-flop circuit 1 storing high-level data on the data line D side and that the select signal m-2 falls and the select signal m-1 rises. The (m-1)th flip-flop circuit 1 holds low-level data on the data line D side. In this case, a charge stored in the parasitic capacitance C coupled to the data line D charges the parasitic capacitance C1 via the select transistor Q5 (R7) of the (m-1)th circuit. Further, a charge stored in the parasitic capacitance /C of the (m-1)th circuit charges the parasitic capacitance /C coupled to the data line /D via the select transistor Q6 (R8). Simultaneously, the function of charging the parasitic capacitance C1 via the resistor R11 in the (m-1)th circuit and the function of charging the parasitic capacitance /C1 via the resistor R10 are activated. The mutual function of the above charging and discharging determines whether or not the state of the (m-1)th flip-flop circuit 1 is inverted. As has been described, the parasitic capacitances C and /C are relatively large, and therefore function as power sources until most of the charges stored therein disappear. In this case, if the resistances R7 and R8 are relatively small, charging from the parasitic capacitance C to C1 via the resistor R7 and discharging from the parasitic capacitance /C to /C1 via the resistor R8 become stronger than charging from the power supply VCC to the parasitic capacitance /C via the resistor R10 and discharging of the parasitic capacitance C1 to the ground via the resistor R11. Hence, the charges (information) stored in the parasitic capacitances C1 and /C1 become equal to the charges (information) stored in the parasitic capacitances C and /C. This corresponds to a change in the stored information, and generates a read error when the next read operation on the (m-1)th flip-flop circuit 1 is performed. With the above in mind, the voltage switch 37 selects the internal voltage VL which is enough large to perform the read operation and which sufficiently increases the internal resistances Q5 and Q6 of the select transistors Q5 and Q6. Hence, the charging and discharging via the resistors R7 and R8 can be suppressed. In the write operation, it is desired that information stored in the (m-1)th flip-flop circuit 1 (charges stored in the parasitic capacitances C1 and /C1) are easily influenced by information of the data lines D and /D (charges stored in the parasitic capacitances C and /C). Hence, it is necessary to facilitate charging from the parasitic capacitance C to C1 and discharging from the parasitic capacitance /C1 to C. Hence, in the write operation, the voltage switch 37 selects the power supply voltage VCC, whereby the gate voltages of the select transistors Q5 and Q6 are increased to the power supply voltage VCC. Fig. 14 is a waveform diagram of the read operation of the circuit shown in Fig. 13 . In the read operation, the select signal applied to the select transistors Q5 and Q6 are equal to the internal voltage VL lower than the power supply voltage VCC. Hence, the internal resistances R7 and R8 of the select transistors Q5 and Q6 are increased and charging and discharging performed via these transistors are suppressed. Hence, the state of the (m-1)th flip-flop circuit 1 cannot be inverted. Fig. 15 is a circuit diagram of the internal voltage generator 38. As shown in Fig. 15 , the internal voltage generator 38 is comprised of P-channel MOS transistors Q53, Q54 and Q55, N-channel MOS transistors Q56, Q57 and Q58, and two resistors R13 and R14. A voltage divided by the resistors R13 and E14 is applied to the gate of the transistor Q56. The constant internal voltage VL is output via the drain of the transistor Q55. If the internal voltage VL is decreased, current flowing in the transistors Q54 and Q57 is decreased, and the drain potential of the transistor Q54 is increased. The drain of the transistor Q54 is connected to the gates of the transistors Q53 and Q54 (so as to form a current-mirror circuit), and hence the current flowing in the transistor Q53 is decreased in accordance with increase in the drain potential of the transistor Q54. Hence, the drain potential of the transistor Q56 is decreased. The decrease in the drain voltage of the transistor Q56 decreases the gate potential of the transistor Q55 and increases the drain potential of the transistor Q55. That is, the decrease in the internal voltage VL can be suppressed. Fig. 16 is a circuit diagram of the decoder circuit 39 shown in Figs. 12 and 13 . The decoder circuit 39 corresponds to the column decoder (Fig. 8) in the DRAM device (or a row decoder in a SRAM device). In Fig. 16, the address signal (the predecoded address signal) consists of four bits /A0, A0, /A1 and A1 for the sake of simplicity. NOR gates G5 through G8 are connected to address bit lines to which the address bits /A0, A0, /A1 and A1 are applied. These NOR gates G5 through G8 are supplied, as power supply voltage, with either the internal voltage VL generated by the internal voltage generator 38 shown in Fig. 15 or the power supply voltage VCC via the voltage switch 37. A description will now be given of an embodiment of the present invention. As has been described with reference to Fig. 4, the sense buffer in the DRAM device or the sense amplifier in the SRAM device is formed with a current-mirror type read circuit made up of MOS transistors Q13 - Q22. It is known that such a current-mirror type read circuit has the best AC input/output characteristic (capable of operating at the highest speed) when the input signals D and /D are approximately equal to VCC/2. However, the conventional structure is designed to stabilize the operations of the flip-flop circuits 1 by precharging the data lines D and /D to the power supply voltage VCC. Hence, the read circuit 5 cannot be operated at the best input/output characteristic. That is, it is impossible to operate the read circuit 5 at high speed. For the above reason, conventionally, a potential shift circuit as shown in Fig. 17 is provided between the data line pair and the sense buffer in the DRAM device (sense amplifier in the SRAM device) in order to select the potentials of the data lines which make it possible to operate the read circuit with its best input/output characteristic. The conventional potential shift circuit shown in Fig. 17 includes transistors Q71 - Q73 and Q81 - Q83. Complementary input signals VIX and VIZ are applied to the gates of the transistors Q72 and Q82, respectively. Complementary output signals VOX and VOZ are output via the drains of the transistors Q73 and Q83, respectively. Fig. 18A shows a circuit configuration in which transistors Q84 and Q74 controlled by select signals  and / are added to the potential shift circuit shown in Fig. 17. These transistors Q74 and Q84 form current paths. Fig. 18B is an equivalent circuit of the circuit shown in Fig. 18A in which a current path occurs on the output VIX side in response to the rise of the select signal  and hence a potential difference between the input signals VIZ and VIX. A description will now be given, with reference to Fig. 18B, of a mechanism in which the complementary output signals VOZ and VOX of the potential shift circuit shown in Fig. 17 are attenuated with respect to the complementary input signals VIZ and VIX. On the VIZ side, the following equations stand: VIZ = VCC x (R2 + R3)/(R1 + R2 + R3) VOZ = VCC x R3/(R1 + R2 + R3) Hence, a shifted potential level Δ VZ is written as follows: Δ VZ = VIZ - VOZ = R2/(R1 + R2 + R3) On the VIX side, the following equations stand: VIX = VCC x (R2 + R3)R4/{R1(R2 + R3) + R4(R1 + R2 + R3)} VOX = VCC x R3 x R4/{R1(R2 + R3) + R4(R1 + R2 + R3)} Thus, the amount Δ VZ of potential shift is written as follows: Δ VX = VIX - VOX = R2 x R4/{R1(R2 + R3) + R4(R1 + R2 + R3)} The amplitude Δ VI of the complementary input signals is expressed as follows: Δ VI = VIZ - VIX = VCC x R1(R2 + R3) 2 / [(R1 + R2 + R3){R1(R2 +R3) + R4(R1 + R2 + R3)}] The amplitude Δ VO is written as follows: Δ VO = VOZ - VOX = VCC x R1R3(R2 + R3)/ [(R1 + R2 + R3){R1(R2 + R3) + R4(R1 + R2 + R3)}] It will now be assumed that A is: A = VCC x R1/[(R1 + R2 + R3){R1(R2 + R3) + R4(R1 + R2 + R3)}] By using the above A, the following equations can be obtained: Δ VI = (R2 + R3) 2 x A Δ VO = R3(R2 + R3) x A It can be seen from the above equations that Δ VO is attenuated with respect to Δ VI, and the amount Δ VIO of this output attenuation is as follows: Δ VIO = R2(R2 + R3)A When R2 is set to be zero in order to reduce the attenuation, Δ VZ = Δ VX = 0 in the equations (1) and (2). This means that the potential shift cannot be performed. As described above, the conventional potential shift circuit shown in Fig. 17 has a disadvantage in that the potential shift is performed with an attenuation of the complementary output signals with respect to the complementary input signals. Fig. 19 is a circuit diagram of an embodiment of the present invention intended to overcome the above disadvantage. A potential shift circuit according to the embodiment of the present invention includes the aforementioned P-channel MOS transistors Q71 and Q81, and the aforementioned N-channel MOS transistors Q72, Q73, Q82 and Q83. However, connections among these transistors shown in Fig. 19 are different from those shown in Fig. 17. The gate and drain of the transistor Q72 are connected together, and are connected to the gate of the transistor Q83. The gate and drain of the transistor Q82 are connected together, and are connected to the gate of the transistor Q73. The gates of the transistors Q71 and Q81 are grounded. In other words, the source of the transistor Q73 (Q83) is grounded. Further, the drain of the transistor Q73 (Q83) outputs the output signal with respect to one of the two input signals, and the gate thereof receives the other input signal. Fig. 20A shows a circuit configuration in which current path transistors Q74 and Q84 are added to the potential shift circuit shown in Fig. 19 . Fig. 20B is an equivalent circuit of the potential shift circuit shown in Fig. 19 in which a current path occurs on the VIX side in response to the rise of the select signal , and hence a potential difference between the inputs VIZ and VIX occurs. It will be noted that transistors Q71 and Q81 functioning as load transistors and the transistors Q72 and Q82 for determining the amount of the shifted potential can be replaced by simple resistance elements. In this case, the same effect as those in the circuit shown in Fig. 20A can be obtained. That is, as shown in Fig. 20B , a series circuit having resistors R1 and R2 is connected to the transistor Q73 in series. The equivalent circuit shown in Fig. 20B . differs from the convention equivalent circuit shown in Fig. 18B in that the resistors R3 used in the conventional circuit are replaced by variable resistors, and the resistance value R3Z of the variable resistor on the VIZ side is different from the resistance value R3X of the variable resistor on the VIZ side. On the VIZ side, the following equations stand: VIZ = VCC x (R2 + R3Z)/(R1 + R2 + R3) VOZ = VCC x R3Z/(RI + R2 + R3Z) Hence, the amount Δ VZ of potential shift is as follows: Δ VZ = VIZ - VOZ = R2/(R1 + R2 + R3Z) On the VIX side, the following equations stand: VIX = VCC x (R2 + R3X)R4/{R1(R2 + R3X) + R4(R1 + R2 + R3X)} VOX = VCC x R3X x R4/{R1(R2 + R3X) + R4(R1 + R2 + R3X)} Hence, the amount Δ VX of potential shift can be written as follows: Δ VX = VIX - VOX = R2 x R4/{R1(R2 + R3X) + R4(R1 + R2 + R3X) The amplitude Δ VI of the complementary input signals is: Δ VI = VIZ - VIX = VCC x R1{(R2 + R3Z)(R2 + R3X) + R4(R3Z - R3X)} ÷ [(r1 + r2 + r3Z) {R1(R2 + R3X) + R4(R1 + R2 + R3X)} The amplitude Δ VO of the complementary output signals is: Δ VO = VOZ - VOX = VCC x R1{R3Z(R2 + R3X) + R4(R1 + R2)(R3Z - R3X)/R1} ÷ [(R1 + R2 + R3Z){R1(R2 + R3X) + R4(R1 + R2 + R3X)}] It will now be assumed that B is defined as: B = VCC x R1 ÷ [(R1 + R2 + R3Z)({R1(R2 +R3X) + R4(R1 + R2 + R3X)}]. Then, the following equations are obtained: Δ VI = (R2 + R3Z)(R2 + R3X) + R4 (R3Z - R3X)} x B Δ VO = {R3Z(R2 + R3X) + R4(R1 + R2) x (R3Z - R3X)/R1} x B The difference between the input signal amplitude and the output signal amplitude is calculated as follows: Δ VIO = VI - VO = R2{(R2 + R3X) + R4/R1 x (R3Z - R3X)} x B It can be seen that equation (6) is equivalent to equation (3) under the condition that R3Z = R3X = R3. The equation (6) shows the difference between the input signal amplitude and the output signal amplitude, the following stands: Δ VIO > 0 : attenuation Δ VIO = 0 : attenuation = amplification = 0 Δ VIO < 0 : amplification The above can be identified by using the following: R2{(R2 + R3X) - R4/R1 x (R3z - R3X)} x B < 0 From the above inequality, the following conditions can be obtained: R3Z < (1 + R1/R4)R3X + R1R3/R4: attenuation R3Z = (1 + R1/R4)R3X + R1R3/R4: attenuation = amplification = 0 R3Z > (1 + R1/R4)R3X + R1R3/R4: amplification When the potential shift circuit according to the embodiment of the present invention is applied to a differential amplifier in the DRAM devices, R1 is the internal resistance of the load transistors Q7 and Q8 (Fig. 6) connected to the data buses, R4 corresponds to the internal resistances of the column selecting transistors Q5 and Q6 (Fig. 6). In the SRAM devices, R1 is the internal resistances of the transistors Q7 and Q8 (Fig. 6), and R4 is the internal resistances of the word transfer transistors Q5 and Q6. In either case, the condition R1 << R4, the attenuation and amplification much depend on the difference between the resistance R3Z and the resistance R3X. Fig. 21 is a block diagram of a DRAM device having the potential shift circuit 44 according to the present invention. In Fig. 21, parts that are the same as those shown in the previously described figures are given the same reference numerals as previously. The potential shift circuit 44 according to the present invention is connected between the sense buffer 15 and the pair 13 of bus lines. Since the DRAM device shown in Fig. 21 employs the potential shift circuit 44, the pair 13 of bus lines can be precharged to the power supply voltage VCC (see Fig. 5). This precharging is performed by a bus line amplitude limit circuit 45 shown in Fig. 21. An internal voltage generator 46 generates an internal voltage necessary for the operation from the power supply voltage VCC. Fig. 2 2 is a block diagram of an SRAM device having the potential shift circuit 44 according to the present invention. In Fig. 22 , parts that are the same as those shown in the previously described figures are given the same reference numerals as previously. The potential shift circuit 44 is connected between the sense amplifier 15 and the pair 13 of bit lines. Since the SRAM device employs the potential shift circuit 44, the pair 13 of bit lines can be precharged to the power supply voltage by the bus line amplitude limit circuit 45. Fig. 23 is a circuit diagram of an essential part of the DRAM device shown in Fig. 21. In Fig. 23, parts that are the same as those shown in the previously described figures are given the same reference numerals as previously. The signals PSA and NSA for driving the sense amplifiers 11 are generated by the circuit shown in Fig. 11. The circuit shown in Fig. 23 can be used for the SRAM device. In this case, the signals PSA and NSA are fixed to the power supply voltages VCC and VSS, respectively. Fig. 24 is a waveform diagram of the operation of the DRAM device shown in Fig. 21 . The select signal m shown in Fig. 24 is generated by the corresponding column decoder 22 in the DRAM device (when the mth flip-flop circuit 1 is selected). In the SRAM device, the select signal m is generated by the related row decoder 21. The select signal m can be generated by, for example, the circuit shown in Fig. 16 . The potential shift circuit shown in Fig. 19 functions to shift down the high-level input potential to the low-level output potential.. It is also possible to provide a potential shift circuit having the function of shifting up a low-level input potential to a high-level output potential. Fig. 25 is a circuit diagram of a potential shift circuit which shifts up a low-level input potential to a high-level output potential. In Fig. 25 , P-channel MOS transistors Q75 and Q85 are substituted for the N-channel MOS transistors Q72 and Q82 shown in Fig. 19. Further, the N-channel MOS transistors Q74 and Q84 are replaced by P-channel MOS transistors Q76 and Q86. The input signal VIZ is applied to the gate and drain of the transistor Q75, the drain of the transistor Q73, and the gate of the transistor Q81. The input signal VIX is applied to the gate and drain of the transistor Q85, the drain of the transistor Q83 and the gate of the transistor Q71. Fig. 26A shows a circuit configuration in which transistors Q76 and Q86 are added to the circuit shown in Fig. 25 . Fig. 26B is an equivalent circuit of the circuit shown in Fig. 26A obtained when the driving signal  falls. The transistors Q71 and Q81 function as variable resistors RIZ and RIX, respectively. By applying the potential shift circuits according to the present invention to DRAM or SRAM devices, it becomes possible to prevent attenuation of the output amplitude caused during the level shifting operation or to amplify the output amplitude. Further, the potential shift circuits are placed on the input sides of differential amplifiers, it becomes possible to increase the operation speed of the differential amplifiers and to contribute to facilitating the read operation of the semiconductor memory devices. Finally, each of the memory cells 10 used in the DRAM devices has, for example, a circuit configuration shown in Fig. 27A . Each of the memory cells 35 used in the SRAM devices has, for example, a circuit configuration shown in Fig. 27B . The present invention is not limited to the specifically disclosed embodiment , and variations and modifications may be made without departing from the scope of the present invention.

Claims: A semiconductor memory device comprising: a flip-flop circuit (1; 11) ; a switch (2; 12) provided between the flip-flop circuit and a pair of data lines; (D,/D; VIX, VIZ) a write circuit (6, 16) writing data into the flip-flop circuit via the switch; and potential shift means (44) connected to the pair of data lines, for converting potentials of the pair of data lines into particular potentials, the potential shift means comprising a first and a second transistor (Q73, Q83); the first transistor (Q73) has a gate connected to the first data line (VIX), a source receiving a first power supply voltage (Vss), and a drain outputting one of said particular potentials (VOZ) obtained by converting the potential of the second data line (VIZ); the second transistor (Q83) has a gate connected to the second data line (VIZ), a source receiving said first power supply voltage (Vss), and a drain outputting another one of said particular potentials (VOX) obtained by converting a potential of the first data line (VIX), characterised in that said potential shift means (44) further comprises: a first series circuit having a first end connected to the drain of the first transistor (Q73), and a second end receiving a second power supply voltage (Vcc); and a second series circuit having a first end connected to the drain of the second transistor (Q83), and a second end receiving the second power supply voltage (Vcc), the first series circuit having first and second resistance elements (Q71, Q72) connected in series, the second series circuit having third and fourth resistance elements (Q81, Q82) connected in series, a node at which the first and second resistance elements are connected in series being connected to said second data line (VIZ) and a node at which the third and fourth resistance elements are connected in series being connected to said first data line (VIX). The semiconductor memory device as claimed in claim 1, characterized in that the first power supply voltage (Vss) differs from the second power supply voltage (Vcc). The semiconductor memory device as claimed in claim 1, further comprising a read circuit (15) which comprises a current-mirror type read circuit. The semiconductor memory device as claimed in claim 1, further comprising dynamic type memory cells (10) connected to the flip-flop circuit, so that the semiconductor memory device functions as a DRAM device in which the flip-flop functions as a sense amplifier. The semiconductor memory device as claimed in claim 1, characterized in that the flip-flop circuit functions as a static type memory cell (35), so that the semiconductor memory device functions as an SRAM device.

Inventors: UCHIDA TOSHIYA, UCHIDA, TOSHIYA, UCHIDA, TOSHIYA, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1994
Main classification: G11C7
IPC codes: G11C7